The present invention relates to an automatic transmission which comprises a power transmission mechanism incorporating a plurality of power transmission paths and a plurality of hydraulically operated frictionally engaging means. These hydraulically operated frictionally engaging means are controlled to select these power transmission paths individually, by means of hydraulic oil which is supplied through a plurality of shift control valves.
Such automatic transmissions have been known and are utilized, for example, as automatic transmissions for vehicles. Typically, an automatic transmission designed for use in a vehicle operates such that the transmission automatically controls the actuation of hydraulic clutches to change the speed change ratio in correspondence to the driving condition of the vehicle. Generally, the automatic transmission includes a speed change hydraulic unit which comprises a plurality of shift control valves, a solenoid valve to control the actuation of these shift control valves, and a manual valve operated in correspondence to the movement of the shift lever manipulated by a driver. In this arrangement, the automatic transmission automatically performs speed change control for a plurality of ranges, i.e., a reverse drive range, a neutral range and a forward drive range (D range, Second, First, etc.) which are individually selected by the operation of the manual valve operated in correspondence to the manipulation of the shift lever (generally, this automatic control is effective only in the forward drive range).
Recently, another type of automatic transmission has been introduced. This automatic transmission does not use a manual valve for the selection of a range and controls the shift of speed change ratios in each range solely by electrical signals. Such transmissions are disclosed in Japanese Laid-Open Patent Publication Nos. H5 (1993)-209683 (A) and H5 (1993)-215228 (A). Each automatic transmission disclosed there comprises a plurality of solenoid valves to control the actuation of shift control valves. The selection of the ranges, i.e., the forward drive range, the neutral range, and the reverse drive range, as well as the selection of the speed change ratios in the forward drive range are performed in correspondence to the command signals (electrical signals ) which actuate these solenoid valves.
In such a speed change control system, as the selecting or switching of drive ranges and the setting of speed change ratios are controlled solely by the control pressures which are supplied from solenoid valves, for example, the switching of drive ranges is controlled simply by the manipulation of the shift lever. However, this simplicity presents a serious problem that when the shift lever is manipulated from the forward range through the neutral range to the reverse drive range (this manipulation is referred here as xe2x80x9cD-N-R manipulationxe2x80x9d), a power transmission path for a reverse drive may be set even while the vehicle is driving forward.
As a preventive measure against such an adversity, a prior-art automatic transmission has been equipped with a reverse inhibitor, which functions to prevent the transmission from shifting into the reverse drive range when a D-N-R manipulation is carried out while the vehicle is driving forward. In other words, the speed of the vehicle must decrease to a predetermined level for the transmission to shift into the reverse drive range. However, a control system of the above described type, which does not include a manual valve, establishes a neutral range solely by a combination of command signals which actuate solenoid valves. It is important or necessary for the reverse inhibitor function of the system to be activated or deactivated in correspondence to the momentary condition of the transmission when the neutral range is to be established. The inclusion of such reverse inhibitor control makes the control system complex, and if a solenoid valve malfunctions while the neutral range is established, there is a possibility that a speed change ratio for the reverse drive range may be selected accidentally.
It is an object of the present invention to provide a control system for an automatic transmission, which system offers a reverse inhibitor function simply and reliably if a D-N-R manipulation is carried out while the vehicle is driving forward.
Another object of the present invention is to provide a control system for an automatic transmission, which system can prevent the transmission whose reverse inhibitor function is activated, from shifting into the reverse drive range in an event that a solenoid valve malfunctions.
In order to achieve these objectives, the present invention provides a control system for an automatic transmission which comprises a power transmission mechanism (for example, the parallel shaft speed change mechanism TM of the preferred embodiment described in the following section), a plurality of hydraulically operated frictionally engaging means (for example, the LOW clutch 11, the SECOND speed clutch 12, the THIRD speed clutch 13, the FOURTH speed clutch 14 and the FIFTH speed clutch 15 of the preferred embodiment) and a plurality of shift control valves (for example, the first shift valve 60, the second shift valve 62, the third shift valve 64, the fourth shift valve 66, the fifth shift valve 68, the CPB valve 56 and the D inhibitor valve 58 of the preferred embodiment). The power transmission mechanism incorporates a plurality of power transmission paths, and the hydraulically operated frictionally engaging means are activated by the supply of hydraulic oil regulated by the shift control valves for the selection of the power transmission paths individually in the power transmission mechanism. This control system comprises a plurality of solenoid valves (for example, the firstxcx9cfifth on/off solenoid valves 81xcx9c85 ) for supplying and draining a line pressure. In this arrangement, the line pressure supplied and drained from the solenoid valves are used to actuate the shift control valves for selecting the power transmission paths in speed change control. For controlling the selection, a plurality of combinations of on/off operations of the solenoid valves are switched to select the power transmission paths individually so as to establish a forward drive range, a neutral range and a reverse drive range in response to a shift manipulation (a manipulation of the shift lever). In this system, the neutral range comprises a first neutral mode and a second neutral mode which are set in response to the shift manipulation carried out to establish the neutral range. The first neutral mode is set if the vehicle incorporating this automatic transmission is driving at a speed equal to or more than a critical speed, and the second neutral mode is set if the vehicle is driving at a speed less than the critical speed. In this case, the combination of on/off operations of the solenoid valves used for setting the first neutral mode is different from that used for the second neutral mode.
In this control system, it is not desirable that the transmission be shifted to the reverse range while the first neutral mode is taking place. Therefore, the system offers a reverse inhibitor function which forbids the transmission to shift into the reverse range while it is in the first neutral mode. On the other hand, while the transmission is in the second neutral mode, the transmission can be shifted either to the forward range or to the reverse range. As the pattern of on/off operations of the solenoid valves is set differently for the first neutral mode and for the second neutral mode, the transmission can be controlled simply and securely to shift into a driving mode which includes this inhibitor function or into a mode which does not include the inhibitor function. Therefore, it is preferable that when the first neutral mode is once set, the system retain the combination of on/off operations of the solenoid valves for setting the first neutral mode as long as the vehicle speed is equal to or more than the critical speed even if a shift manipulation to the reverse range is carried out by the driver. In this way, the system can provide a perfect inhibitor function.
It is preferable that the system further comprise a forward/reverse hydraulic servomechanism (for example, the forward/reverse selection hydraulic servomechanism 70 of the preferred embodiment) for selecting either the power transmission paths which belong to the forward drive range or the power transmission paths which belong to the reverse drive range. In this case, preferably, the system be constructed such that while the transmission is in the first neutral mode, the forward/reverse hydraulic servomechanism selects the power transmission paths of the forward drive range. With this construction of the control system, as the forward/reverse hydraulic servomechanism selects the power transmission paths of the forward drive range while the transmission is in the first neutral mode, even if, for example, a solenoid valve malfunctions electrically or a shift valve malfunctions mechanically, there is no possibility for any power transmission path of the reverse drive range to be established.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.